The Chemistry of Hair Colors
Yesterday, I did a talk for a cosmetic science class at the University of Toledo which outlined the hair research I did which eventually resulted in this patent. It was all about the process of how we went through some basic research to determine why colored hair fades and ways to prevent that from happening. It was fun to remember that time. While putting together the research I had to review the chemistry of hair colors. I noticed that we’ve never really written about that here on Chemists Corner so I figured now would be as good a time as any to do it.
Types of hair color
Natural hair color is the result of two types of melanin pigments, eumelanin which is responsible for the brown and black colors of hair and pheomelanin which creates the orange or blonde hues. Together, these two molecules are responsible for every hair color on the planet, except for the artificial ones or grey hair which is the absence of any pigment.
There are a number of options for synthetic hair color and these are classified by the types of color molecules used and the length of time that they last. They include the following
- Temporary hair color
- Semi-permanent / demi-permanent hair color
- Permanent hair color
- Bleaching
We’ll go through each of these and explain how they work.
Temporary hair color
Temporary hair colors are ones that are meant to be applied and worn for only a short amount of time. They are great for
occasions when you just want to try out a new color. They also have many more color options than you can get with most other hair colors. Here’s a picture of when I tried a pink colored temporary hair color. The thing about temporary hair colors is that they only coat the surface of the hair (they can’t penetrate) so they are easily removed with one or two shampooings. The colorants used are acid or basic dyes. Many of them are the same colorants used for food. Acid dyes are more easily removed because they are less compatible with hair. Basic dyes may be slightly more substantive but they too a readily removed.
Semi-permanent hair color
Semi-permanent hair colors can penetrate the surface of the hair into the cuticle layer. These products will last for a few more washings than temporary colors but they too will eventually be washed out. The vast majority of dyes used for semipermanent colors include nitrophenylenediamines, nitroaminophenols and aminoanthraquinones. The first two compounds create yellow to violet colors while the last provides violet to blue hues. Semi-permanent hair colors work great for people who just want to experiment with a new color. They also work well for grey hair coverage. One of the challenges for semi-permanent colors is that they do not completely cover the natural hair color so this tends to limit the color pallet that is available for the consumer. That also means hair is not as damaged but it’s a trade off.
Bleaching
When a consumer wants to go lighter in color, one way to permanently do that is to bleach the hair. Beaching essentially is an oxidation reaction with the hair melanin that causes it to lose color. Strong bleaching requires a combination of ammonia, hydrogen peroxide and ammonium persulfate. This will properly open up the hair shaft and break down the melanin. Hair color is typically described on a 12 point scale with a 12 being ultra blonde and a 1 being black. The maximum level of bleaching you can achieve with one treatment is a change of a 6-7 level. Also, once this bleaching is done the hair is permanently changed in color. New hair at the roots will be the natural hair color but the bleached hair will remain bleached unless otherwise colored.
Permanent hair coloring
The most common hair color is permanent hair coloring. This process involves a change in hair color that is “permanent” or at least until new hair grows. The process involves a couple of steps including bleaching out the natural hair color (by 3-4 levels) and adding the new color. The dyes used are actually dye precursors. These small molecules are monomers which are able to penetrate into the hair all the way to the cortext. Common compounds used include p-phenylenediamine and p-aminophenol. Permanent hair color is a three step process that begins with colorless monomers.
- Oxidation of the monomer to a reactive species via peroxide
- Addition of a coupler to give a dye intermediate
- Oxidation of intermediate to create the final dye
This is a polymerization reaction so the dye molecules become too large to easily come out of the hair shaft upon washing. Thus, you get a permanent coloring.
The significant issues with this type of coloring is that it damages the hair structure and you have limited colors that can work. Also, a patch test needs to be done to ensure that the person getting the hair colored does not have a negative reaction.
So, there you have it. Hair coloring in a nutshell. Of course, there are many more details but we’ll save that for a future post.
